Ian,
Can you be more specific on how the Projects are created by interact? Are these Email to Case or specific to the creation of Projects?
Technically, this would not be done by Interact but by a Workflow Rule that would run on projects that 'on creation'. Then the action would be Update Field.
For further clarification, we suggest you register for one of our Advanced Q&A webinars (see the link-> http://www.clarizen.com/resources/online-demo.html#second ).These are great resources for free consulting with our Customer Success Managers and visual answers to your questions.
Please let us know if you need further assistance.
Sincerely,
Boris